Mainly, there are two types of laminated panels: so-called high pressure panels and low pressure panels.
For many years now, laminates have been used as materials for dwellings and industrial and commercial buildings. Typical applications of such laminates are for covering the surfaces of furniture, table tops, chairs and the like. Consequently, they are subjected to much friction and to abrasion; and therefore, they are required to have a high resistance to abrasion.
The so-called high pressure laminates are produced from a core which is constituted of resin-impregnated sheets. Said sheets are generally sheets of kraft paper: which have been impregnated with a thermosetting resin and more particularly a phenolic resin.
Once the sheets have been impregnated with resin, they are dried, cut and stacked one over the other. The number of sheets in the stack varies with the applications and may be between 3 and 9, and even more.
Then a decorative sheet is placed over the stack of sheets constituting the core. Said decorative sheet is generally a sheet of paper carrying a printed or colored pattern, or comprising decorative particles, said sheet being impregnated with a thermosetting resin which does not blacken under heat, such as for example melamine formaldehyde resins, benzoguanamine formaldehyde resins, non-saturated polyester resins. Normally, a protective covering sheet without any pattern and which remains transparent in the final laminates, is placed over the decorative sheet. In the trade, said protective sheet is called overlay. The stack of impregnated sheets is then placed inside a press equipped with a metal plate conferring the surface finish. Then the stack is densified by heating and pressing to obtain a uniform structure.
The so-called low pressure laminates are produced in the same way as the high pressure laminates, except that the decorative sheet is directly laminated onto a panel of wood particles or any other supporting base.
The protective sheet or overlay is conventionally produced by draining an aqueous suspension of virtually non-refined cellulose fibers. Said sheet has a low basis weight and is not opacified. It is impregnated with thermosetting resin, which makes it transparent in the final laminate and reveals the decorative pattern of the latter.
Said overlay sheet protects the surface of the laminate, and in particular it increases its resistance to abrasion through an extra intake of thermosetting resin.
This is the most common way to increase the resistance to abrasion of laminates.
This resistance, however, can be further increased by using abrasive particles in or on the overlay or on the decorative sheet proper.
For example, patents FR-A-2 104 707, FR-A-2 139 990, U.S. Pat. No. 3,661,673, U.S. Pat. No. 5,141,799 and CA 836522 have described the use of particles, hence of near-spherical shape, of alumina also known as corundum, of silica, of boron carbide or of silicon, titanium or tungsten.
Said particles may be incorporated in bulk or placed on the surface of the sheet or even deposited on the resin-impregnated sheet.
Said particles present two disadvantages which are that, on the one hand they cause premature wearing out of the machines on which are produced the protective sheets containing them, and of the laminating presses, and that on the other hand, the bonding of said particles in or on the sheets is not easy due to the fact that either they are difficult to keep in bulk or they show poor adherence to a surface.
To overcome said disadvantages it is conceivable to use abrasion-resistant fibers which, because of their fibrous nature, should be more readily integratable with the cellulose fibers.
However, Patent GB-A-1 201 071, filed in 1967, reports in its description of the prior art that the incorporation of glassfibers in a protective sheet with a view to improving the resistance to abrasion of laminates, has not given satisfactory results, particularly as the fibers have been found to affect the appearance of the laminate.
The comparative examples described hereinafter have indeed confirmed that said fibers remain visible in the sheets after impregnation.
Said examples further reveal that the glassfibers used in ground form give erratic results.
As shown in the already cited patents, this idea has since been abandoned and all the research has been directed to the use of particles, hence of rounded form, and more or less abrasive, and to the various methods of incorporating them, in or on the protective sheet or the decorative sheet.